Normally, light waves vibrate in a large number of planes about the axis of a light beam. Upon reflection off or transmission through certain materials, the waves can be made to vibrate in one plane only, in which case, the light is said to be plane (or linearly) polarized.
While several naturally-occurring materials inherently possess light polarizing properties, synthetic polarizing materials based on thin polymeric films are desirable for their comparative ease of manufacture and handling and their ready ability to be tailored for particular uses. Among the most common synthetic polarizers are dichroic polymers comprising aligned long-chain chromophoric polymer molecules. Light entering such a dichroic polarizer encounters two different absorption coefficients, one low and one high. Light emerges linearly-polarized, vibrating in the direction of the low absorption--a direction generally correspondent with the direction of alignment of the polarizer's chromophores.
The development of such synthetic dichroic polarizers has made practical the widespread utility of light polarizing elements for a variety of applications. For example, in the manufacture of electrooptical devices, such as liquid crystal display screens, a pair of polarizers are used in conjunction with an addressable liquid crystal interlayer to provide the basis for image formation. The pair of polarizers are positioned with either their respective polarizing axes crossed or parallel. Depending on the electrically-selectable molecular orientation of the liquid crystal interlayer, polarized light passed through the first polarizer of the pair can be "twisted" into or out of alignment with the polarizing axis of the second polarizer, thus either blocking or admitting the transit of light therethrough.
Flat panel liquid crystal displays are highly desirable because--in contrast with typical cathode ray tube displays--they occupy less volume, are lighter, and require less power. They can be made quite compact, and thus, highly portable. Still, though good flat panel liquid crystal displays are available, efforts continue in the development of a flat panel liquid crystal display that is inexpensive and durable, yet capable of displaying bright, well-resolved, high contrast images.
In this effort, attention is directed to the influence of the optics and polarizing properties of conventional polarizers on the overall display properties of a liquid crystal display. While polarizing efficiency is needed for good display resolution, the polarizing element used in a liquid crystal display also absorbs some of the light used for image production, thus reducing contrast and dimming the displayed image.
Dimness can of course be countered by more greatly illuminating the display. However, enhancing the brightness of a display's back- or edge-lighting components, for example, can increase power requirements, thus compelling the use of heavier, bulkier batteries. With such compromise to portability being undesirable, an alternative solution would be to employ more efficient polarizing elements, ones that continue to polarize light well, but at greater yields.
Unfortunately, the most direct way to obtain higher efficiencies is to increase the concentration of a polarizer's dichromophore. But, as is well known to those skilled in the art, an increase in dichromophore concentration unavoidably results in a lower photopic transmittance level: Essentially, as polarization efficiency increases, luminous transmittance (Kv) decreases.